This invention relates to devices to oscillate a test sample, and more specifically, to a device that oscillates a test sample for use in determining the density of fluids using a hollow oscillator filled with the sample under test. The method and apparatus hereof can be used to impart and maintain
Devices to measure the density of a fluid mixture by oscillating the mixture within a container (i.e.; oscillator) and observing the period of oscillation are known. These devices typically include four main parts: an oscillator, an actuator, a support, and a countermass. The actuator powers the oscillation of the oscillator, which is typically filled with the fluid to be tested. A support holds the oscillator for oscillation, and a countermass provides a stable surface with respect to which the oscillator is oscillated. An example of such a device is shown in Stabinger, et al U.S. Pat. No. 5,477,726, discussed more fully below.
Several techniques of accomplishing the above are taught in the prior art. A magnet may be attached to the oscillator and driven by nearby fixed coils as taught, for example, by Janssen, U.S. Pat. No. 3,728,893. Alternately, the coil wire may be mounted on the oscillator and the magnet fixed as taught by Albert, et al in U.S. Pat. No. 4,655,075. The oscillator itself may be made of a conducting material so as to serve as a single turn coil driven by a fixed magnet as disclosed by Herrero-Alvarez in Rev. Sci. Instrum. 68 (10), October 1997.
In these techniques the actuator or a part of the actuator is mounted directly on the oscillating element. This can have number of practical disadvantages:
First, heat generated in the actuator is readily transmitted to the sample under test. Because the density of the sample under test is generally a function of temperature, an error is introduced.
Second, the mass of the portion of the actuator attached to the oscillator adds to the oscillating mass of the system. Increasing the total oscillating mass relative to the mass of the sample under test within reduces the sensitivity of the device. Additionally, as taught by Kratky et al in U.S. Pat. No. 3,910,101, non-linearity in the relation of density to the square of the period grows with the ratio of oscillating mass to counter mass. Physically, this happens when motion induced into the counter mass moves the nodes that define the volume of the sample under test.
It is common practice to calibrate density measuring instruments with two known samples, often dry air and water. With only two calibration points any non-linearity compromises the accuracy of the measurement of densities that differ from the calibration points. Consequently an increase in the total oscillating mass requires either a much greater increase in the counter mass of the system to preserve accuracy or a further means of compensating for insufficient counter mass as disclosed, for example, by Kratky.
Third, it is advantageous to control the environment surrounding the oscillator by enclosing it within a hermetically sealed housing. Having actuator components within the housing complicates manufacture and repair.
One possible means to avoid the practical disadvantages listed above is to impart the vibratory motion not to the oscillator itself, but rather to a member to which the oscillator is attached. An example of this was disclosed by Muramoto in U.S. Pat. No. 4,132,110 in which a piezoelectric actuator introduces motion into a member attaching the ends of two parallel oscillating tubes. Beneficially, the mass of the actuator does not contribute to the mass of the oscillators. However, mounting the actuator on connector between the oscillators still creates a short thermal path into the sample under test. Also of interest in this device is the fact that the two oscillators move in opposition thereby eliminating the need for a counter mass.
A later example of introducing the vibratory motion through a member to which the oscillator attaches is taught in Stabinger, et al U.S. Pat. No. 5,477,726. Two embodiments are disclosed. In the first, the actuator is mounted between the counter mass and a thermostatically controlled housing within which the oscillator is supported. In the second embodiment the housing is eliminated and the actuator acts directly on the support of the oscillator. As in Muramoto, connecting the actuator to the support of the oscillator creates a path for heat from the actuator to flow readily into the sample under test unless further temperature control means are employed.
Although the first embodiment of Stabinger does prevent heat generated in the actuator from reaching the sample under test, another aspect of the apparatus is consequently compromised. The mass driven relative to the counter mass, consisting of the housing, temperature control means, and the oscillator supported within, is very large. For a bench top installation, where the total mass must be limited, a high ratio of oscillating mass to counter mass cannot be achieved. Consequently, the entire apparatus is in motion and provides no convenient mounting points which can be fixed without risk of influencing the oscillation. Mechanical suspensions or other further means to control and compensate for the resulting motion of the counter mass are then required.
Stabinger teaches that the temperature control means, which in his preferred embodiments are Peltier devices, are to be placed between the actuator and the oscillator. To function effectively, Peltier devices, which essentially are heat pumps, must be in intimate thermal contact with a large thermal reservoir such as the counter mass. Mounting the Peltier devices between the oscillator and actuator isolates the devices from the counter mass. Heat must then be pumped through the actuators. Piezoelectric actuators are constructed of ceramic materials and have a low thermal conductivity. Consequently, the thermal performance of this configuration is compromised.
A further disadvantage of both embodiments of Stabinger is that the entrance and exit ports of the oscillating tube translate with the actuation of the oscillator. This requires that any ancillary equipment connected to the ports be sufficiently compliant so as not to influence or restrict the action of the actuator. Vibratory motion imparted to equipment connected to the moving ports can also excite parasitic resonant vibration into the attached equipment. Parasitic resonance is a known source of error in the art.
In light of the disadvantages of the prior art, there exists a need for a density measuring apparatus that is free from temperature induced error, that is inherently linear in measurement, that can be effectively temperature controlled and that can be conveniently mounted to fixed objects such as benching, piping or other accessory equipment.